"Having a moon is just inherently cool. And it is something that most self-respecting planets have, so it is good to see that this one does too."

Xena, known formally as 2003 UB313, is nicknamed after the warrior princess of television fame. And its moon Gabrielle is named after Xena's TV travelling companion.

The pair live beyond Pluto in a band known as the Kuiper Belt, a region that is home to comets, asteroids and other space rocks.

The possible 10th planet moves in a highly eccentric orbit, tilted some 45° above the orbital plane of the other planets.

Its orbit is also elliptical, zooming in as close as 5.6 billion kilometres from the Sun and moving out to as far as 14.5 billion kilometres away.

Earth orbits rather consistently at 150 million kilometres from the Sun.

It takes Xena 560 Earth years to complete one trip around the Sun, compared with Pluto's 250 years.

Xena is one of three big planet-like bodies recently found in this region. The others have equally playful nicknames: Santa and Easterbunny.

A planet or not?

Size is important when it comes to making the grade as a planet. Astronomers know that Xena is bigger than Pluto but since they don't know what it is made of, they can't be sure that it is more massive.

The discovery of the moon Gabrielle means Xena has at least enough mass to keep a satellite.

Gabrielle is estimated to orbit close to Xena, making a circuit perhaps every 14 days. It is about 60 times fainter than Xena.

The International Astronomical Union, which decides on what is a planet, considers Xena a trans-Neptunian object, meaning its orbit crosses that of Neptune, just as Pluto's does.

Many astronomers, including Brown, also question Pluto's planetary status.

But Xena's discovery, and its size, have prompted the union to rethink the definition of planet.

"The very rapid pace of discovery of bodies within the solar system over the last decade, and so our understanding of the Trans-Neptunian Region is therefore still evolving very rapidly. This is in serious contrast to the situation when Pluto was discovered," the union says on its website.

A working group of the union is considering a new definition. Until the group finishes its work, the website statement says, all objects discovered at a distance of 40 times Earth's distance from the Sun "will continue to be regarded as part of the Trans-Neptunian population".